In the recovery of oil from subterranean reservoirs, one of the more successful methods employed to-date is miscible flooding, which involves injecting a solvent into the formation to dissolve oil and facilitate its efficient extration from the reservoir. When the solvent injected into the formation is capable of forming a single phase with the reservoir fluid at formation conditions immediately on contact therewith, the condition is referred to as instant miscibility.
Miscible flooding is a very effective oil recovery process for removing oil from subterrean reservoirs. By creating a single phase system in the reservoir, the retentive forces of capillarity and interfacial tension, which cause a significant reduction in recovery by non-miscible flooding processes, are eliminated. Furthermore, the mixing of the injected fluid with the formation oil reduces the viscosity of the oil, as a result of which the oil flows or can be displaced more efficiently through the permeable oil reservoir.
While hydrocarbons, e.g. paraffinic hydrocarbons in the C2 to C6 range have been employed successfully in miscible flooding, these materials are quite expensive and the cost of a miscible flood employing a substantial amount of light hydrocarbons is exceedingly high. Carbon dioxide has also been used successfully as an oil recovery agent. Carbon dioxide is a particularly desirable material because it is highly soluble in oil, and dissolution or carbon dioxide in oil causes a reduction in the viscosity of the oil and increases the volume of oil, all of which improve the recovery efficiency of the process. Carbon dioxide is sometimes employed under non-miscible conditions, and in certain reservoirs it is possible to achieve a condition of miscibility at reservoir temperature and pressure between essentially pure carbon dioxide and the oil.
More recently, prior art references have recognized the fact that carbon dioxide may be employed as a recovery agent under conditions in which only conditional miscibility is achieved at reservoir conditions. Conditional miscibility is distinguished from instant miscibility by the fact that miscibility between the injected carbon dioxide and the reservoir petroleum is achieved sometime after the first contact between carbon dioxide and the reservoir petroleum, as a result of a series of transitional multi-phase conditions, wherein the injected fluid vaporizes intermediate hydrocarbon components from the reservoir petroleum to form a mixture of carbon dioxide and intermediate hydrocarbon components, with the concentration of intermediate hydrocarbon components increasing with time as the bank moves through the reservoir until a miscible condition is achieved in situ as a consequence of the contact between the injected fluid and the reservoir petroleum.
When the fluid injected into the reservoir is gaseous at reservoir conditions, injection conditions must be controlled carefully to achieved efficient displacement even if conditional miscibility can be achieved. This relates to the fact that gaseous displacing fluids ordinarily are inefficient displacing agents under many conditions encountered in subterranean reservoirs. If the reservoir itself is a dipping reservoir, i.e., if the angle between the reservoir and the horizontal plane is greater than 5.degree. and preferably greater than 10.degree., stable conditions can be achieved if the gaseous fluid is injected up-dip to displace the petroleum in a downward direction, so long as the linear velocity of the injected bank through the formation does not exceed a critical velocity value. The critical velocity is proportional to the formation permeability, the difference in density between the displacing and displaced fluid, and the sine of the dip angle of the formation, and is inversely related to the mobile fluid porosity and the difference in viscosity between the displaced and displacing fluid. Since carbon dioxide is a highly compressible gas, the density of gaseous carbon dioxide under many reservoir conditions is nearly equal to the density of liquid formation petroleum, and so the density difference is quite low. The low density difference means the critical velocity required to insure maintenance of a stable displacing front is very low, and so the fluid injection rate must be maintained at a level too low for economical operating conditions. While prior art references teach the dilution of carbon dioxide with inert gas to reduce the density of the injected fluid, addition of inert gas to carbon dioxide reduces the miscibility of the fluid, and in critical situations can result in changing the injected fluid from one which is conditionally miscibile with the formation petroleum, to one which is no longer conditionally miscible.
In view of the foregoing discussion, it can be appreciated that there is a significant need for an oil recovery method employing carbon dioxide under conditions of conditional miscibility where the conditional miscibility is maintained after the density difference is increased to permit flooding at a reasonably high rate so as to insure that the oil recovery process is economically feasible.